1. Field of the Invention
The present invention relates to a communication system having a management server function and performing communication by a token access method or a communication system in which host computers to which subsystems are connected are connected to each other through a wide-area network, and electronic mail communication is performed between the subsystems connected to the host computers.
2. Description of the Prior Art
FIG. 2 shows a ring-coupled LAN (Local Area Network) of a general token access method.
FIG. 2 illustrates a state wherein medium access control is performed by a so-called token access method in which communication control apparatuses A to E each called a node are logically coupled in a ring shape, and only an apparatus having a token as a communication right assignment command has the right of transmission. In FIG. 2, five nodes, i.e., A to E are connected by a logic ring. For example, for the node C, the node B serves as an upstream node, and the node D serves as a downstream node.
Such a LAN is generally called a token ring, and is a world-wide standard as defined by ISO.8802/5. For this reason, its detailed description will be omitted. Network management in this LAN is basically performed as follows.
Each node performs own-node management, and a device called a network management server arranged in a given node acquires each management information. A network manager grasps/understands the acquisition result, and performs corresponding processing as needed.
Note that management information herein means, e.g., logging information, statistical information, format information, and the like.
However, some user systems are not equipped with a network management server. In general, each node incorporates a simple network management server function to acquire information within a limited range.
As a conventional acquisition means of management information, each node transmits a command requesting for transfer of intra-node management information of other nodes to the other nodes by multi-address calling periodically or as needed, and sequentially receives corresponding management information from the other nodes.
However, in a user system without a network management server, since each node randomly requests management information of other nodes, many management packets are generated in the LAN. For this reason, transfer efficiency (information volume transferred in a unit time) of user data is decreased.
A wide-area network system using a LAN system has a similar problem.
FIG. 17 shows a wide-area network system using the general LAN system.
In FIG. 17, four intra-area subsystems A 30, B 40, C 50, and D 60 are connected to a wide-area network 20 through host computers A 31, B 41, C 51, and D 61, respectively.
Each of the intra-area subsystems 30 to 60 is constituted by the LAN shown in FIG. 2, and a plurality of terminals T 32 to 65 are connected to the corresponding host computers A to D (31 to 61).
Each terminal can perform electronic mail communication with a terminal in another intra-area subsystem connected to another host computer through its own host computer and the wide-area network 20. For example, when the terminal 34 in the subsystem A 30 sends an electronic mail to the terminal 65 in the subsystem D 60, electronic mail transmission is executed in the following procedure.
First, the terminal 34 requests an electronic mail registered user list in the subsystem D 60 to the host computer A 31, and obtains the electronic mail registered user list obtained by a method to be described later. The terminal 34 determines the address value of the terminal 65 on the basis of the obtained electronic mail registered user list, and sends an electronic mail having the determined address value as a header to the host computer A 31.
The host computer A 31 recognizes on the basis of the header that the electronic mail is addressed to a terminal in the subsystem D 60, and adds a header addressed to the host computer D 61 to the electronic mail and sends it onto the wide-area network. The packet received at the host computer D 61 is sent to the terminal 65 via the LAN in the subsystem D 61.
In order to smoothly operate an electronic mail system, addresses of electronic mail registered users managed by each host computer must include not only those in its own subsystem but also those in all the subsystems connected to the wide-area network.
More specifically, the host computer A 31 must manage not only addresses of in the subsystem A 30 but also addresses of all the terminals in the subsystems B 40, C 50, and D 60. The addresses to be managed must always be the latest addresses in correspondence with all changes in system.
As a conventional method of updating an address table, the following two methods are known.
In a first method, a manager of the entire system obtains latest address tables from managers of the subsystems, and stores them in a recording medium such as an MT (magnetic tape), and sends the record by mail to the host managers of the subsystems periodically (once a month), so that the manager of each host computer updates its own address table.
A second method is performed by inter-host communication utilizing the wide-area network. In this method, a manager of, e.g., the host computer A 31 transmits the latest address table of the subsystem A 30 to all the remaining host computers B 41, C 51, and D 61, so that each host computer updates the address table associated with the subsystem A 30.
However, the first and second conventional methods have the following problems.
In the first method, since an updating cycle is long, a new user who wants registration must wait for a long period of time until the address table is updated, and cannot enjoy electronic mail service during this period.
If the updating cycle is shortened, the manager of the entire system and the managers of the subsystems are overloaded.
In the second method, the number of communications required for updating is increased, and as a result, communication fee charged to users is increased.
For example, when host computers are scattered at 100 locations, 9,900 inter-host communications are required for all the host computers to complete updating of the entire system.